Very slowly varying electromagnetic waves can penetrate not just air, but also earth and water. This is true despite the large conductivity of earth and water (e.g., the conductivity of sea water is about 4 A/Vm). Hence, electronic communication can be achieved through earth and water as well as through air. For instance, it is known that underwater communication to depths of 500 m can be achieved with sinusoidal waves of 50 Hz and less. However, two problems are encountered when communicating with such low frequency sine waves. First, at such low frequencies, antennas of one quarter wavelength and other effective designs are very long (i.e., 1 km to 100 km). Hence, they are not very portable and may even be required to be permanently located on large tracts of land. A second problem is that efficient sine wave antennas have small relative bandwidths. With a typical relative bandwidth of 1%, a carrier frequency of 50 Hz will only be useful in transmitting signals with a baseband bandwidth of about 0.5 Hz. Since Nyquist s theorem states that the information rate can not exceed two pulses per cycle, a maximum of only one pulse per second can be transmitted (i.e., 2.times.0.5 Hz=1 pulse per sec.). Since teletype characters consist of a sequence of five pulses, five seconds are required to transmit one teletype character. Accordingly, one line of 60 characters would require 300 seconds (i.e., five minutes). It is evident that, in the above situation the transmission rate of information is extremely slow. However, the transmission rate can be increased to approximately 50 pulses per second, (i.e., a speedup of 50 times), by using a large relative bandwidth and eliminating the requirement for a sinusoidal carrier. By using pulses with a duration of 20 ms and occupying the entire frequency spectrum from 0 to 50 Hz (i.e., 0&lt;f&lt;50 Hz) a transmission rate of 50 pulses per second (i.e., 10 teletype characters per second) can be achieved. However, there remains the problem of designing a high current antenna for such slowly varying waves, particularly for mobile use.
U.S. Pat. No. 4,506,267 to H. F. Harmuth describes the antenna that is the predecessor to the present invention, and which is an efficient high current radiator for large relative bandwidth signals. It has a forward loop geometry that is very different from the return loop, and has a high permeability shield around the return loop that absorbs its electromagnetic radiation. That antenna, however, is for pulses with a duration of approximately one nanosecond, and, as described in that patent, is not an effective radiator for pulses on the order of 10 milliseconds. For such relatively long pulses, ferrites cannot be used for separating the forward and the return loops of the radiator, other materials and techniques are needed.
The high current capability of the above mentioned radiator enables us to make an antenna that has a short length. As mentioned above, the long length of effective antennas was always a problem for slowly varying electromagnetic waves. Radiators for slowly varying electromagnetic waves are characterized by the product sI, where s is the length of the radiator and I the peak current flowing through the radiator. This fact allows a trade-off between s and I. That is, one can build physically small radiators by using large antenna currents, or vice versa. But large currents (e.g., hundreds of kiloAmperes) present a problem that has heretofore been a major obstacle.
Accordingly, one object of this invention is to provide a highly efficient antenna for radiating low frequency non sinusoidal electromagnetic energy with wide relative bandwidth.
Another object is to provide such an antenna which can handle large currents.
Another object of this invention is to increase the efficiency of an electromagnetic radiator by introducing a specially designed shield that reflects electromagnetic waves.
A further object of this invention is to arrange the geometry of the radiator and the composition of the shield so that the electromagnetic shield absorbs the electromagnetic waves radiated by return loop current, and reflects the electromagnetic energy from a forward loop, thereby increasing the efficiency of the antenna.
Yet another object of this invention is to provide a material that has low conductivity in the direction of the incident electric field vector and high permeability in the direction of the incident magnetic field vector, which will act as a reflector for electromagnetic energy.
Another object of the invention is to provide an antenna in which the antenna current is n times greater than the drive current, by configuring the current path of the antenna as a series wound transformer with n loops, driven by the drive current.
Another object of this invention is to fabricate an antenna with an antenna current that is so large that the antenna itself can be made small and portable even for very low frequency radiation.
Still a further object of this invention is to make an antenna for low frequency electromagnetic waves that is small enough that it can be portable, for carrying on shipboard, airplane, or automobile.
A further object of this invention is to show how an efficient high current land based radiator of great length can be built up by combining many small high current radiators into a large coordinated system.